The invention relates to a pre-engraved substrate for a memory disk recording information in response to a stress of a the magneto-optical or magnetic or phase-change type. This stress is applied to the first surface of the substrate, and not through it, as is the case in the traditional optical disks. Moreover, this stress will only be applied to the parts of the substrate between the pre-engraved zones, these parts being often called xe2x80x9clandsxe2x80x9d in the art. It relates also to a process for manufacturing this substrate as well as to the recordable memory disk which has this substrate or which is obtained by this process.
Currently, recordable memory disks or diskettes are widely used, for example, in computers as computer hard disks.
These disks or diskettes, in their most evolved versions, function in first surface recording mode and are magnetic disks with optical guiding assistance also called optically assisted magnetic disks, magneto-optical disks with optical guiding assistance, or phase-change optical disks.
They are generally composed of a medium that has a pre-formatting signal, and of a single- or multi-layer structure constituting the recordable memory and deposited on this medium, after engraving of pre-formatting in the medium.
The single- or multi-layer structure of the recordable memory has a layer in which the information items to be stored in memory are recorded.
In this type of memory, contrary to conventional optical disks, recording in the recordable layer is achieved by direct application, on the first surface, that is to say, without the passing through the medium, of the appropriate stress.
This stress can be applied by a magnetic field, thermal energy supplied by a light beam, or a combination of a magnetic field and thermal energy. In the disks of the magneto-optical and magnetic type, the recording of the information is done in the recordable layer by creation of zones in which the magnetic field is oriented differently (reversed) with respect to the magnetic field of the other zones, and in the phase-change optical disks, the recording is done by creation of zones in which the material constituting the recordable layer has undergone a phase transformation, that is to say has gone from an amorphous structure to a crystalline structure or from a crystalline structure to an amorphous structure.
Reading the information is then done by detection of the different magnetic fields or the differences in reflectivity due to the structural change of the material, respectively.
In all cases, the recordable layer consists of a hard material that is magnetic or capable of phase-change.
Disks of this type allow one to reach very high storage capacities, but their performances are still limited by alteration of the signal-to-noise ratio of the pre-format read signal, also called C/N ratio, generated by deposition of the layer or layers constituting the recordable memory, on the pre-format engraved at the beginning of the substrate manufacturing cycle.
The invention aims to alleviate this problem by proposing a recordable memory disk of the magneto-optical, magnetic or phase-change optical type having an improved signal-to-noise ratio of the pre-format read signal and whose manufacturing is simpler and more economical than the disks of this type currently manufactured.
To this effect, the invention proposes a pre-engraved substrate for a memory disk recordable by a stress of the magneto-optical or magnetic or phase-change optical type which has (a) a medium that has, on its surface, a layer made of a reflective material allowing one to obtain a surface with an optical polish, (b) on this layer of reflective material, a single- or multi-layer structure that has at least one layer recordable by application to the first surface of the substrate of a stress of the magnetic, magneto-optical or phase-change optical type, the layer being of reflective material, and the single- or multi-layer structure having a series of grooves and/or micro-pits representing a pre-formatting signal and extending from the surface of the outermost layer of the single- or multi-layer structure to a predetermined depth of the layer of reflective material that is less than the thickness of this layer of reflective material, and (c) optionally, a protective layer deposited on and entirely covering the last layer of the single- or multi-layer structure, the lateral walls of the grooves and/or micro-pits being formed, in each of the layers of the layer of reflective material and of the single- or multi-layer structure, only of the material constituting these layers, disregarding the aforementioned protective layer.
According to a first embodiment of the pre-engraved substrate of the invention, the medium and the layer of reflective material allowing one to obtain a surface with an optical polish are composed of different materials.
The medium can, for example, be made of glass or aluminum, and the layer of reflective material allowing one to obtain a surface with an optical polish can, for example, be made of nickel, brass, aluminum, chromium or titanium nitride.
According to a second embodiment of the pre-engraved substrate of the invention, the medium and the layer of reflective material are made of the same material.
In this case, the medium and the layer of reflective material are advantageously formed as a single piece.
An example of a material that can be used in this second embodiment is aluminum.
In all the embodiments of the invention, examples of materials constituting said at least one recordable layer are Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te, Thxe2x80x94Fexe2x80x94Co, Thxe2x80x94Fexe2x80x94Cr, Thxe2x80x94Fexe2x80x94Coxe2x80x94Cr, Tbxe2x80x94Gexe2x80x94Sb, Thxe2x80x94Gexe2x80x94In, Thxe2x80x94Gexe2x80x94Ag, Fexe2x80x94Cr, Fexe2x80x94Co, or their alloys and mixtures.
The invention also relates to a process for manufacturing a pre-engraved substrate which includes:
(a) a step, performed first, of deposition of a single- or multi-layered structure that has at least one layer, recordable by application to the first surface of the substrate of a stress of the magnetic, magneto-optical or phase-change optical type, on the surface layer, made of reflective material that allows one to obtain an optical polish of a medium, (b) a step, performed second, of engraving a series of grooves and/or micro-pits representing a pre-formatting signal in the single- or multi-layered structure and in the surface layer of reflective material, the engraving extending from the outermost layer of the single- or multi-layered structure to a predetermined depth of the surface made of reflective material that is less than the thickness of this surface layer of reflective material, and (c) an optional step, performed last, of deposition of protective layer 11 on the surface of the last layer of the single- or multi-layered structure.
According to a preferred mode of implementing the process of the invention, the step of engraving the series of micro-pits and/or grooves representing the pre-formatting signal includes the steps of:
deposition of a photosensitive resin layer on the outermost layer of the single- or multi-layered structure;
recording of the pre-formatting signal in the form of a latent image in the photosensitive resin layer by scanning with a light beam or exposure through a mask;
elimination of the resin zones thus transformed;
transfer by appropriate engraving of the pre-formatting signal into the layers constituting the single- or multi-layered structure and the surface layer of reflective material of the medium down to a predetermined depth of the surface layer of reflective material that is less than the thickness of this surface layer of reflective material, and;
elimination of the residual resin.
In a first mode of implementation of the process of the invention, the medium consists of a different material from that of the surface layer of reflective material.
Thus, the medium can be made of glass or aluminum, and the surface layer can be made of nickel, brass, aluminum, chromium or titanium nitride.
In a second mode of implementing the process of the invention, the medium of [sic; and] the surface layer of reflective material are made of the same material.